Not what you're asking but used one-handed Maltron keyboards can often be had for little money on ebay UK. There's one for 45 Pounds at the moment. Bit clunky a

Not what you're asking but used one-handed Maltron keyboards can often be had for little money on ebay UK. There's one for 45 Pounds at the moment. Bit clunky and creaky though. Personally I'd get a Kinesis Advantage with a footswitch. Use one half of they keyboard as is, and the footswitch to change the layout.